This book is developed from notes I used in a course at Princeton that attempted to bring database systems into the mainstream of computer science. The course was taught to a mix of seniors and first-year graduate students. In it, I tried to relate database ideas to concepts from other areas, such as programming languages, algorithms, and data structures.. A substantial amount of descriptive material was included, since students, being used to conventional programming languages, may find query languages rather unusual. The data structures relevant to databases are also somewhat different from the kinds of structures used in conventional programming, since the large scale of a database makes practical many structures that would be only of theoretical interest otherwise.